The 'costs' of showing Iran that the US is determined ...'

(Begin here with the Benefits,)

"... Costs:
In addition to the financial costs of conducting military attacks against Iran,
which would be significant (particularly if the U.S. had to carry out thousands of sorties and if it had to return to the use of force periodically for years to come), there would likely be near-term costs associated with Iranian retaliation, through both direct and surrogate asymmetrical attacks. Serious costs to U.S. interests would also be felt over the longer term, we believe, with problematic consequences for global and regional stability, including economic stability. A dynamic of escalation, action, and counteraction could produce serious unintended consequences that would significantly increase all of these costs and lead, potentially, to all-out regional war.
Among the potential costs discussed in this paper are the following:
*Direct Iranian retaliation against the U.S.
While some argue that Iran might hold back using force in order to avoid provoking a larger scale conflict, we believe that Iran would retaliate, costing American lives; damaging U.S. facilities in the region; and affecting U.S. interests in Iraq, Afghanistan, the Gulf, and elsewhere. Iran would draw on its
extensive conventional rocket capability and IRGC anti-ship missiles, small submarines, fast attack boats, and mine warfare in the Gulf. Iran might attempt to close the Strait of Hormuz, which could rattle global markets and cause a significant spike in oil prices (as well as blocking the main artery for export of Iran’s own oil).
*Iranian strikes against Israel. Iran would hold Israel partly responsible for any
attacks, whether or not Israel participated in military action. While Israel's
anti-missile and civilian defense programs are strong, sustained air strikes by Iran would result in casualties and damage to facilities, perhaps including the Israeli nuclear complex in Dimona.
*Indirect retaliation by Iran. Attacks by well-armed proxies such as Hezbollah
or Shiite militant groups in Iraq, as well as by Iran’s covert forces and the IRGC
Qods Force, could be even more damaging to U.S. and Israeli interests than direct Iranian retaliation. Such indirect retaliation could include the use of missiles and rockets by proxies as well as terrorist attacks and covert action, such as sabotage and assassination. If Hezbollah were to make heavy use of the missiles and rockets it has deployed in southern Lebanon, that could expand the conflict, possibly leading to a regional war in the Levant.
*A potential breakdown of hard-won global solidarity against Iran’s nuclear
program. We believe that if Iran is attacked by the US or Israel  in the absence of an international mandate or a multinational coalition, support for
support for  to Iran that are now prohibited by sanctions could resume, as might the sale of materials that could be used for making a nuclear weapon. 

*Global political and economic instability, including disruptions in energy supply and security. A U.S. and/or Israeli attack on Iran could introduce
destabilizing political and economic forces in a region already experiencing major transformations. In addition to costing the U.S. economy hundreds of billions of dollars yearly, a sustained conflict would boost the price of oil and further disrupt an already fragile world economy
*Damage to the United States’ global reputation and increased credibility for anti-American extremist groups. U.S. military action, especially if unilateral, could further alienate Muslims and others worldwide, reinforcing the view that the United States resorts too often to military force. An attack on a Muslim nation could enhance the recruiting ability of radical Islamist groups, including Al Qaeda. Even though some Sunni Muslims might be pleased to see attacks on Shiite Iran, the likely impact on U.S. stature in the Muslim world would be negative.
On the contested issue of whether military action would weaken or strengthen
Iranian public support for the current regime, we conclude that U.S. and/or Israeli strikes are more likely to unify the population behind the government than to generate resistance.Some of these costs would be mitigated if a U.S. strike were to occur in response to Iranian actions that clearly revealed an intention to develop a nuclear weapon. Such actions might include the expulsion of IAEA inspectors and withdrawal from the NPT, or the launch of a crash program to raise existing supplies of low- and medium-enriched uranium to a weapons-grade level of enrichment. Given the time required for Iran to progress from the decision to weaponize to possession of a reliable, deliverable weapon, the United States would have an opportunity to develop international support for multilateral action against Iran, including further sanctions, additional negotiations, and the use of military force. While the costs associated with Iranian retaliation would not significantly be altered if other nations approved or joined in a U.S. military strike, the longer-term costs to U.S. interests would be somewhat lessened...."